New Conformations and Binding Modes in Halogen-Bonded and Ionic

tetragonal polymorph of the free donor, and obeying Ostwald's rule of stages, decomposes through solid-state loss of I2 to give exclusively this p...
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New Conformations and Binding Modes in Halogen-Bonded and Ionic Complexes of 2,3,5,6-Tetra(2′-pyridyl)pyrazine Clifford W. Padgett,† Rosa D. Walsh,† Gregory W. Drake,‡,# Timothy W. Hanks,§ and William T. Pennington*,†

CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 2 745-753

Department of Chemistry, Clemson University, Clemson, South Carolina 29634-1905, Air Force Research Laboratory, Edwards Air Force Base, California 93524-7680, and Department of Chemistry, Furman University, Greenville, South Carolina 29613 Received August 2, 2004;

Revised Manuscript Received September 7, 2004

ABSTRACT: The enormous potential of 2,3,5,6-tetra(2′-pyridyl)pyrazine (tppz) as a versatile multisite component for the design and construction of complex structures is demonstrated by the preparation of several new complexes that possess previously unobserved binding modes and conformations. tppz forms halogen-bonded complexes with diiodine and organoiodides and forms ionic compounds in which tppz is either di- or tetraprotonated. We have prepared two new halogen-bonded complexes of tppz with diiodine and with tetraiodoethylene (TIE). Both possess extended chain structures of alternating donors and acceptors, with the tppz donors in the diiodine complex linked by two neutral -I2‚I2‚I2- chains. Two new polyiodide complexes of tppz have also been prepared, one of which has di- and the other tetraprotonated cations. A bromide salt of the tetraprotonated tppz cation has also been prepared and found to be isomorphous with the known chloride salt. In the diiodine complex, tppz‚6I2, the conformation of tppz is similar to that found in the metastable, tetragonal polymorph of the free donor, and obeying Ostwald’s rule of stages, decomposes through solid-state loss of I2 to give exclusively this polymorph. In the TIE complex, tppz‚TIE, has a previously unreported conformation, but decomposes to the thermodynamically stable monoclinic polymorph upon loss of TIE. The diprotonated cation, [tppz(H)2]2+, which forms two intramolecular N-H‚‚‚N hydrogen bonds and crystallizes with (I2‚I3)- anions, has a twisted conformation as opposed to the bowed conformation found for this cation with tetraphenylborate counterions. Tetraprotonated tppz hydrogen bonds to two iodide anions to form [tppz(H2I)2]2+ cations, similar to the bromide and chloride salts, but with triiodide anions rather than hydrated halides as in the lighter derivatives. Introduction In the preceding paper in this issue,1 computer analysis of the conformational behavior of 2,3,5,6-tetra(2′-pyridyl)pyrazine (tppz) was reported. The enormous potential of this molecule as a multisite node for the design and construction of complex structures was also discussed. While the use of tppz as a ligand with a variety of metal centers has been vigorously pursued, its utilization in other systems has been modest. In this paper, we report our initial efforts to probe the capacity of tppz for crystal design. The preparation of two halogen-bonded tppz complexes and one diprotonated tppz salt, each possessing previously unobserved binding modes, confirms the promise of this molecule as a versatile component for the construction of complex materials. To highlight similarities to hydrogen bonding, the term “halogen bond” has been coined to describe the interaction of an electron pair donor with a halogen atom, either bonded to carbon or to another halogen (X-X′ or X2).2-4 More recently, due to their strength, selectivity, and directional nature, halogen bonding in * Corresponding author. William T. Pennington, Department of Chemistry, Clemson University, Clemson, SC 29631-0973. Phone: 864656-4200; fax: 864-656-6613; e-mail: [email protected]. † Clemson University ‡ Air Force Research Laboratory. § Furman University. # Present address: Propulsion Research Center, NASA Marshall Space Flight Center, Alabama 35812.

the solid state has proven to be a valuable tool for crystal design.5-8 We have taken advantage of this interaction to prepare complexes in which small Lewis acid molecules act as removable structure controlling vectors. In one study,9,10 we prepared two different complexes of tppz with elemental iodine. Both were found to be metastable, decomposing through solid-state loss of iodine. As a free molecule, tppz crystallizes as two conformational polymorphs.11,12 Through structure analysis, we found that the conformation of the tppz molecule in tppz‚2I2 was very similar to that in the thermodynamically stable monoclinic polymorph of tppz, and decomposition of the complex led exclusively to that form. Suitable single crystals of the other complex could not be obtained at that time due to the extreme ease with which iodine is lost from the material. Interestingly, this species led to the metastable, tetragonal polymorph of tppz after complete removal of iodine. On the basis of computational analysis, we suggested that the conformation of the tppz molecule in the molecular complex is similar to that in the tetragonal polymorph. In this paper, we report the successful structure solution of this complex and confirm that this is indeed the case. The formation of infinite one-, two-, or three-dimensional networks is an important goal in crystal design, due to the collective chemical and physical properties that may result from the cooperativity of molecular or ionic interactions. Diiodine is polarized by strong halogen bonding, which reduces its potential as a node for

10.1021/cg049730z CCC: $30.25 © 2005 American Chemical Society Published on Web 10/27/2004

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extended structures. As seen in the two complexes described above, this behavior either leads to simple adduct formation (as in tppz‚2I2) or to the formation of neutral polyiodine chains in which the N-complexed I2 is amphoteric, acting as Lewis acid to the nitrogen donor and as Lewis base to the bridging I2 molecule.13,14 Organoiodides containing two or more iodines are less affected by this polarization, and are more likely to interact at additional acceptor sites to form extended structures. Continuing our work with tppz, we prepared a complex with tetraiodoethylene (TIE), a multisite Lewis acceptor. As reported here, a complex onedimensional chain structure results. Interestingly, the conformation of the tppz molecule is different than that of either polymorph. Prediction of the molecular conformation of tppz in a polar solvent, reported in the preceding paper in this issue,1 was in agreement with the polymorph observed to crystallize from ethanol solution. The conformation of tppz in the polymorph obtained from toluene is similar to the next lowest energy form predicted for the gas phase and nonpolar solvents. The lowest energy form for these media, however, is very similar to that found in the complex with TIE, and efforts to isolate a new crystal form of tppz by decomposition of the complex are described. For some time, we have been interested in the fascinating behavior of iodide salts relative to the other halides.15-17 The I-‚‚‚I2 and I3-‚‚‚I2 interactions within the seemingly endless variety of polyiodide chains, layers, and networks bear obvious similarities to N‚‚‚I and related interactions, and we include them in our definition of halogen bonding. In addition to the neutral complexes mentioned above, we also present the structural analysis of several new ionic complexes of tppz in which the donor is either di- or tetraprotonated. In the diprotonated form, tppz exhibits proton sponge behavior,18,19 similar to a previously reported tetraphenylborate salt,11 but the conformation of the dication is different for the two salts. A pair of tetraprotonated salts are also described, and they provide comparison of the behavior of an iodide salt versus the lighter halides. The iodide salt shows a preference for halogen bonding over hydrate formation that occurs in the lighter halides. The systems reported here serve as limited examples of the versatility and possibilities offered by this exceptional molecule, and provide comparison of its potential as a multisite node for the construction of interesting network solids based on hydrogen- and halogen-bonded systems. Experimental Section Materials and Methods. tppz was purchased from Aldrich Chemical Co. and used as received. Iodine was purchased from Fisher Scientific Co., Fair Lawn, NJ, and was resublimed prior to use. Solvents were obtained from commercial houses and were dried and purified by standard techniques and stored over activated sieves. Carbon, hydrogen, and nitrogen analyses were performed by Atlantic Microlabs, Norcross, GA. Raman spectra were recorded in Pyrex melting point capillaries on a Bruker model FRA 106/S Equinox 55 Raman spectrometer equipped with a 1.06 micron IR excitation laser. Thermal gravimetric analysis was performed under nitrogen gas on a Perkin-Elmer Series 7 analyzer. Sample masses ranged from 5 to 10 mg. All samples were heated at a constant rate of 5 °C/min from 30 °C until all material had vaporized. Preparation of tppz‚6I2. tppz was dissolved in chloroform, and the beaker was sealed in a chamber with a large excess

Padgett et al. of solid iodine. As iodine vapor diffused into the solution, small deep red crystals formed, and grew to a size suitable for X-ray analysis within a few days. This product can also be formed by slow evaporation of a chloroform solution of tppz with excess iodine, but the crystals were too small for data collection. Crystals of tppz‚6I2 lose iodine very quickly; therefore, elemental analysis could be not performed. Preparation of tppz‚TIE. tppz (0.076 g; 0.20 mmol) and TIE (0.105 g; 0.20 mmol) were dissolved in approximately 50 mL of methylene chloride and heated to reflux for approximately 20 min and then filtered. Slow evaporation of the solution yielded light yellow parallelepiped crystals of tppz‚ TIE in 75% yield (0.135 g; 0.15 mmol). Calc. for C26H16N6I4: C, 33.94; N, 9.13; H, 1.75%. Found: C, 33.95; N, 9.03; H, 1.76%. Solid-State Decomposition of tppz‚TIE. Crystalline tppz‚ TIE (0.15 g; 0.16 mmol) was placed in a 250 mL three-neck round-bottom flask outfitted with a sublimation cold finger and vaccum manifold. The flask was gently heated (∼60°) for 2 days under a slight vacuum. Light yellow crystals (0.08 g) which formed on the cold finger were collected, as was the material remaining in the flask. In the latter, the shape of the original crystals was maintained, but they had taken on a powdery, opaque appearance. Preparation of [tppz(H)2](I2‚I3)2 and [tppz(H)4](I)2(I3)2. Reddish black tabular crystals of [tppz(H)2](I2‚I3)2 were grown as a side product in the reaction of tppz and diiodine in ethanol to prepare halogen-bonded complex. Participation of adventitious water apparently led to the ionic compound. A more directed preparation from reaction of tppz with excess hydroiodic acid, also in ethanol, led to a mixture of crystals of this compound and slightly lighter red parallelepiped crystals of the tetraprotonated salt, [tppz(H)4](I)2(I3)2. Neither of these mixtures could be separated to give isolated products suitable for elemental analysis or percent yield. Preparation of [tppz(H)4](Br)4‚2H2O. tppz (0.10 g; mmol) was dissolved in 75 mL of toluene with stirring and heating. An excess of liquid bromine was added to the solution resulting in immediate formation of an orange precipitate. After the sample sat overnight, a small amount of orange crystals of [tppz(H)4](Br)4‚2H2O grew from this solution. Powder diffraction analysis was used to verify that the precipitate and the crystals were identical. Quantum Mechanical Calculations for the Diprotonated tppz Cations. The two conformations of [tppz(H)2]2+ were drawn with geometries similar to those in the crystal structures and minimized using DFT with the B3PW91 (correlation/exchange model) with 6-31**G+ (or 6-31G + (d,p)) basis set with the program, Gaussian 03.20 Charge was set at 2+ with singlet spin. X-ray Diffraction Analysis. Specific details of the crystallographic experiments and results for each compound are given in Table 1. The data were measured at room temperature (295 ( 1 K) with graphite-monochromated Mo Ka radiation (λ ) 0.71073 Å) on either a four-circle Nicolet R3mV diffractometer (tppz‚TIE, [tppz(H)2](I2‚I3)2, and [tppz(H)4](I)2(I3)2) with serial detection or on a three-circle Rigaku AFC8 diffractometer equipped with a Mercury CCD area detector (tppz‚6I2 and [tppz(H)4](Br)4‚2H2O). The data were corrected for Lorentz and polarization effects. An absorption correction, based on either azimuthal scans of several intense reflections (for the serial detector data) or on a multiscan technique (for the CCD data),21 was applied to the data for each compound. All structures were solved by direct methods and refined (on F2) using full-matrix, least-squares techniques. The crystal quality of tppz‚6I2 was poor due to rapid loss of iodine. Several attempts to collect data on crystals of this compound at reduced temperature were unsuccessful due to an apparent phase change that resulted in shattering of the crystal. Only the iodine atoms could be refined anisotropically for this compound. The nitrogen and carbon compounds were refined isotropically. For the remaining compounds, all non-hydrogen atoms were refined anisotropically. Hydrogen atoms for all of the compounds were either refined isotropically (tppz‚TIE) or were included in the structure factor calculation at optimized positions with isotropic displacement parameters fixed at

Complexes of 2,3,5,6-Tetra(2′-pyridyl)pyrazine

Crystal Growth & Design, Vol. 5, No. 2, 2005 747 Table 1. Crystal Data

formula Mw crystal system space group a, Å b, Å c, Å β, (°) V, Å3 Z Dcalc, g cm-3 µ, mm-1 transmission coefficients reflections collected reflections unique (Rmerge) R1a wR2b

tppz‚6I2

tppz‚TIE

C24H16N6I12 1911.23 monoclinic P21/c (No. 14) 15.220(5) 12.982(5) 10.484(4) 99.066(10) 2045.8(13) 2 3.10 9.109 0.48-1.00 15118 3198 (0.20) 0.1302 (0.1383) 0.2747 (0.2792)

C26H16N6I4 920.05 monoclinic C2/c (No. 15) 11.607(4) 13.987(6) 17.854(5) 92.41(2) 2896(2) 4 2.11 4.33 0.79-1.00 2693 2559 (0.027) 0.0261 (0.0301) 0.0609 (0.0618)

[tppz (H)2](I2‚I3)2 C24H18N6I10 1659.44 orthorhombic Pnnn (No. 48) 11.0741(11) 11.6272(13) 15.2196(13) 1959.7(3) 2 2.81 7.93 0.63-1.00 1509 1299 0.0243 (0.0361) 0.0451 (0.0465)

[tppz (H)4](I)2(I3)2

[tppz (H)4](Br)4‚2H2O

C24H20N6I8 1407.66 monoclinic P21/c (No. 14) 11.895(2) 11.682(2) 12.957(2) 94.230(10) 1795.6(5) 2 2.60 6.93 0.67-1.00 2500 2350 (0.021) 0.0233 (0.0368) 0.0397 (0.0406)

C24H24N6O2Br4 748.13 monoclinic P21/c (No. 14) 6.5692(19) 23.521(8) 9.108(3) 101.578(14) 1378.7(8) 2 1.80 5.87 0.62-1.00 12151 2820 (0.036) 0.0756 (0.0777) 0.1282 (0.1291)

a R ) ∑||F | - |F ||/∑|F | for observed data (I > 2σ(I)); number in parentheses is for all data. b wR ) {∑[w(F 2 - F 2)2]/∑[w(F 2)2]}1/2 1 o c o 2 o c o for observed data (I > 2σ(I)); number in parentheses is for all data.

values 20% greater than that of their host atom. Structure solution, refinement, and calculation of derived results was performed with the SHELXTL-Plus package of computer programs.22 Neutral atom scattering factors and the real and imaginary anomalous dispersion corrections were taken from International Tables for X-ray Crystallography, Vol. C.23 Powder diffraction data were acquired on a Scintag XDS/ 2000 theta-theta diffractometer with Cu KR1 radiation (λ ) 1.54060 Å) and an intrinsic Germanium solid-state detection system.

Results and Discussion Conformations of tppz. Conformational differences in the tppz molecule resulting from complexation, whether through protonation or Lewis acid interactions with either metal centers or halogen-bonding acceptors, involve the rotational orientation of the terminal 2′pyridine rings relative to the central pyrazine ring. Differences can be described by the relative positions of the pyridyl nitrogen atom relative to the pyrazine ring plane and by the dihedral angle between the various ring planes. Dihedral angles and conformations, using notation described in our preceding paper in this issue, are reported for the two polymorphs of tppz, the complexes reported here and for several related complexes in Table 2, and are shown in Figure 1 for the two polymorphs and the two halogen-bonded complexes. Specific comparisons for each of the complexes will be discussed in appropriate sections below. Halogen-Bonded Neutral Complexes. Selected geometric parameters for the neutral complexes are given in Table 3. The crystal packing of both complexes are dominated by noncovalent donor-acceptor interactions involving N‚‚‚I, and in the case of tppz‚6I2, I‚‚‚I halogen-bonding interactions. All of these interactions are considerably shorter than accepted sums of van der Waals contact distances for the atoms involved (N: 1.55-1.61 Å; S: 1.79-1.80 Å; I: 1.98-2.00 Å).25,26 tppz‚6I2. This complex crystallizes in the monoclinic space group, P21/c, with the tppz molecule situated about an inversion center at (1/2, 1/2, 1/2). Each pyridyl nitrogen atom is halogen bonded to an I2 molecule. The geometric parameters associated with the N‚‚‚I interaction in tppz‚6I2 are essentially identical to those of the related compounds, tppz‚2I2 and dpq‚I2. In addition to the N‚‚‚I distance being shorter than the sum of van

Table 2. Dihedral Anglesa between Ring Planes and Conformation Notationb in tppz and Its Complexes compound

conformation

A/B

A/Z, B/Z

monoclinic tppz tetragonal tppz tppz‚6I2

62.4° 60.4° 56(1)°

48.9, 51.7° 46.4, 59.0° 48(1), 57(1)°

5NNNN 3XNXN 3XNXN

tppz‚2I2 tppz‚TIE

56.7° 50.8(2)°

56.6, 30.8° 48.2(1), 52.0(1)°

5NNNN 4NNNN

[tppz(H)2](I2‚I3)2

11.1(2)°

18.5(1), 18.5(1)°

c

[tppz(H)2](B(C6H5)4)2 [tppz(H)2]Cl2 [tppz(H)4](I)2(I3)2

35.4°

26.0, 20.7°

c

60.0(2)° 52.9(2)°

59.6, 16.6° 31.5(2), 38.9(2)°

5NNNN 5XXXX

[tppz(H)4]Br4‚2H2O

61.2(3)°

39.9(3), 42.2(3)°

5XXXX

[tppz(H)4]Cl4‚2H2O

60.5(1)°

38.5(1), 41.3(1)°

5XXXX

a

ref 11 12 this paper 9 this paper this paper 11 11 this paper this paper 24

Dihedral angles correspond to the ring planes:

b

As defined in our preceding paper in this issue. c These distorted conformations do not fit any of the conformation classes.

der Waals radii, as noted above, the interaction is essentially linear at the iodine acceptor with an I-I distance that is elongated over that found in elemental iodine (2.715 Å).32 The geometry about the donor nitrogen atom is approximately trigonal planar, with some expansion away from the linkage site of the pyridyl ring. The resulting tppz‚4I2 moieties are linked by I‚‚‚I halogen bonding through bridging I2 molecules (see Figure 2). An inversion center at (0, 1/2, 1/2) relates adjacent complexes in the resulting extended chain, which runs along the a-axis (see Figure 3). The neutral -I2‚I2‚I2- bridges are similar to those found in related complexes with acridine, 9-chloroacridine, and 2,2′bipyridine.13,14 As predicted, the conformation of the tppz molecule in this complex (3XNXN) is very similar to that found in the tetragonal polymorph (see Table 2 and Figure 1). That this complex and tppz‚2I2 each undergo solid-state decomposition (involving diffusional loss of the iodine molecules) to give the polymorph in which the tppz

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Figure 1. Conformations of tppz in the two polymorphs of the free ligand and halogen bonded complexes. The symbol in parentheses describes the conformation using the system described in the preceding paper in this issue.

molecules have the same general conformation is a manifestation of Ostwald’s Rule of Stages,33 which simply stated indicates that an unstable chemical system will often transform directly into the one that most closely resembles it, rather than the most stable of all possible states.

Figure 2. Thermal ellipsoid plot (50% probability) of tppz‚ 6I2 (letters indicate symmetry operator: A, -x, 1 - y, 1 - z; B, 1 - x, 1 - y, 1 - z).

tppz‚TIE. The complex of tppz with TIE crystallizes in the monoclinic space group, C2/c. The tppz molecule sits upon a crystallographic 2-fold axis (0, y, 3/4) which is coincident with the two pyrazine nitrogen atoms. The TIE molecule is situated about an inversion center at

Table 3. Geometric Parameters for Halogen-Bonded Complexes compound

N‚‚‚I (Å)

I-I (Å)

C-N‚‚‚I (°)

N‚‚‚I-I(°)

ref

2.56(3)

2.755(3)

168.4(6)

this paper

2.50(2)

2.760(3)

tppz‚2I2

2.562(8)

2.750(1)

2,3-dipyridyl-quinoxaline‚I2

2.532(3)

2.759(1)

110(2) 133(2) 115(2) 124(2) 111.4(6) 130.6(6) 116.1(3) 125.5(3)

tppz‚6I2

compound tppz‚6I2 (acridine)2‚3I2 (9-Cl-acridine)2‚3I2 2,2′-bipyridine‚3I2 compound

Id‚‚‚Ia (Å)

Ia-I (Å)

I-Id‚‚‚Ia (°)

3.378(4) 3.529(4) 3.475(1) 3.595(4) 3.876(4) 3.550(6)

2.730(3)

106.8(1) 97.2(1) 80.8(1) 100.9(4) 85.5(4) 83.0(1)

2.731(1) 2.720(7) 2.712(5)

N‚‚‚I (Å)

I-C (Å)

2.989(3)

2.095(14) 2.100(16) 2.112(16) 2.16(2) 2.102(3)

3.098(3)

2.105(3)

4,4′-bipyridine‚TIE

2.949(4)

2.110(6)

4,4′-bis-pyridylethylene‚TIE

2.840(6)

2.133(10)

2,2′-bipyridine‚TIE

3.123(7)

2.133(15)

pyrazine‚TIE

2.98(3)

2.11(5)

quinoxaline‚TIE

2.953(9)

2.16(1)

3.136(9)

2.09(1)

3.066(1)

2.102(2)

TIE

tppz‚TIE

phenazine‚TIE

C-N‚‚‚I (°)

174.5(6) 173.0(2)

9

173.8(1)

27

Id‚‚‚Ia-I (°)

ref

178.1(1) 173.2(1) 177.2(1) 175.2(4) 176.0(4) 175.5(2)

this paper 13 13 14

N‚‚‚I-C(°)

ref 28, 29

112.0(2) 117.0(2) 112.7(2) 118.7(2) 104.1(4) 141.4(4) 109.9(6) 134.5(6) 97.3(7) 144.3(7) 121(4) 123(4) 116.0(9) 127.5(9) 114.3(9) 119.9(9) 119.1(2) 123.5(2)

171.5(1)

this paper

170.9(1) 169.1(2)

8

178.7(8)

8

163.2(6)

29

176(1)

30, 31

178.2(7)

29

168.8(7) 173.0(1)

29

Complexes of 2,3,5,6-Tetra(2′-pyridyl)pyrazine

Crystal Growth & Design, Vol. 5, No. 2, 2005 749

Figure 5. Crystal packing of tppz‚TIE viewed down the a-axis, normal to the halogen-bonded chains (origin is the upper, left, rear corner; +x is out; +y is down; +z is right). Figure 3. Crystal packing of tppz‚6I2 viewed down the a-axis, parallel to the halogen-bonded chains (origin is the upper, right, rear corner; +x is out; +y is left; +z is down).

Figure 4. Thermal ellipsoid plot (50% probability) of tppz‚ TIE (letters indicate symmetry operator: A, 1 - x, y, 1.5 - z; B, 1.5 - x, 0.5 - y, 1 - z; C, -0.5 + x, 0.5 - y, 0.5 + z; D, 0.5 + x, 0.5 - y, -0.5 + z).

(1/4, 1/4, 1/2) which lies at the midpoint of the carboncarbon double bond. Infinite chains of alternating tppz and TIE molecules, generated by an n-glide operation perpendicular to the b-axis (x, 1/4, z), are linked through N‚‚‚I halogen bonding, with nitrogen atoms of pyridyl rings in the 2- and 5-positions of the pyrazine interacting with one TIE molecule and those at the 3- and 6-positions interacting with another (see Figure 4). The N‚‚‚I distances are within the range of those for related complexes (see Table 3) and the interactions are linear at the iodine acceptor and approximately trigonal planar at the nitrogen donor. The extended donor acceptor chains are linked in the other two dimensions by edgeface interactions involving the pyridyl rings of tppz. These interactions involve H‚‚‚C distances of 2.90(4)3.02(4) Å, with dihedral angles of 50.8(1) and 79.9(1)° between interacting ring planes. The crystal packing of tppz‚TIE is shown in Figure 5. The conformation of the tppz molecules in this complex (4NNNN) is quite different than that of either polymorph of tppz, and is similar to the one predicted to be most stable in the gas phase. Assuming that the packing interactions of this conformation might not be that different than those of the known polymorphs, we

were hopeful that decomposition would again follow Ostwald’s Rule of Stages and we might isolate a new polymorph of tppz. TGA indicated a 57.4% mass loss at an onset temperature of 204.6 °C, which agrees well with the calculated mass percent of TIE in the complex (57.8%). Powder diffraction analysis of the decomposition products indicated that the diffusion product collected from the cold finger (see Experimental Section) was TIE and that the material remaining in the flask was the monoclinic polymorph of TIE (shown in Figure 6). Although this result strongly suggests that there is no stable crystal form with the conformation found in the TIE complex, the possibility remains that one does or could exist, but that Ostwald’s Rule is not followed in the decomposition process for tppz‚TIE. Given the much larger size of the TIE molecule relative to I2, it is reasonable to assume that its diffusion is much more disruptive to the lattice and that a metastable conformation could not be maintained. Another possibility that must be considered is that the packing interactions of the 4NNNN conformation found in tppz‚TIE might be similar to those of the known conformations. The 4NNNN conformation is, in fact, similar to the 5NNNN conformation found in the monoclinic polymorph, and it is conceivable that they might crystallize in isomorphic fashion to give identical powder patterns. To rule out this possibility, the Raman spectra of the TIE complex was compared to those of the two polymorphs. Raman spectroscopy provides a very reliable characterization technique for distinguishing the structural differences between polymorphic systems as the scattering bands are typically sharp and consequently often contain little spectral overlap.34 As can be seen in Figure 7, the spectrum for the tppz remaining after removal of the TIE matches very well with that of the monoclinic form, indicating that this is undoubtedly the product of tppz‚TIE complex decomposition. It is interesting to note however that a very similar compound, tetraphenylpyrazine, has been found to be dimorphic with conformations related to all three of the low energy forms predicted by our calculations.35 In the R-form of this compound, the molecule sits upon an inversion center and is related to the conformations of tppz in either of its known polymorphic forms. The

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Figure 6. Powder X-ray diffraction patterns: (a) calculated pattern for tppz‚TIE; (b) measured pattern for product remaining after decomposition (loss of TIE); (c) calculated pattern for monoclinic polymorph of tppz. Stars identify peaks associated with tppz‚TIE that had not decomposed.

β-form has an asymmetric unit consisting of two unique molecules, each of which possesses crystallographic 2-fold symmetry and has a conformation similar to that predicted for tppz in the gas phase and found in the TIE complex. Diprotonated Salts of tppz. Diprotonated salts of tppz can form interionic hydrogen bonds to the anion to give complexes of the general form [tppz(H‚X)2], or if N-H‚‚‚X interactions are not favored, intramolecular N-H‚‚‚N bridges can form, allowing the molecule to act as a proton sponge. The chloride salt exhibits the former and the tetraphenylborate salt exhibits the latter behavior.11 For the iodide salt reported below, the interionic interactions cannot compete with the intramolecular bridges, and the latter behavior is observed. For packing stability, additional iodine is needed and the anion is present as a polyiodide (I2‚I3-) chain. [tppz(H)2](I2‚I3)2. The diprotonated salt, [tppz(H)2](I2‚I3)2, crystallizes in the orthorhombic space group, Pnnn. The dication, shown in Figure 8a, is situated about a crystallographic 222 site at (1/4, 1/4, 1/4), coincident with the center of the pyrazine ring. The proton is disordered over two equivalent sites (only one set is shown in Figure 8a), bridging pyridine rings related by a 2-fold operation about (1/4, 1/4, z). The polyiodide anion, shown in Figure 8b, consists of an infinite chain of alternating triiodide anions and diiodine molecules. The triiodide is symmetrical (I1-I2 ) 2.9318(5) Å) with the central iodine atom lying on a crystallographic 2-fold axis at (1/4, y, 1/4). The thermal displacement parameters for the atoms of the anion give no indication of an asymmetrical ion disordered about the symmetry element. The diiodine molecule (I3-I3B ) 2.7742(11) Å) is also situated about a crystallographic 2-fold axis (-1/4, y, 1/4) which is coincident with the midpoint of the I-I bond. The I3-‚‚‚I2 distance of 3.4069(8) Å found in the anion is typical for this class of polyiodide species, and the zigzag conformation of the extended chain is very common.36-39 The polyiodide chains run parallel to the a-axis, and additional weak I‚‚‚I interactions link chains related by 3.7502(10) Å to form loosely associated, slightly corrugated layers, with the cations occupying spaces between the layers (see Figure 9).

Figure 7. Raman spectra for (a) tetragonal tppz; (b) monoclinic tppz; (c) tppz after removal of TIE from tppz‚TIE complex.

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Figure 8. Thermal ellipsoid plot (50% probability) of (a) [tppz(H)2]2+ cation; (b) (I2‚I3-) polyiodide anion (letters indicate symmetry operator: A, 0.5 - x, y, 0.5 - z; B, -0.5 - x, y, 0.5 - z; C, x, 0.5 - y, 0.5 - z; D, 0.5 - x, 0.5 - y, z. Figure 10. Side view of the two different conformations of the [tppz(H)2]2+ cation: (a) [tppz(H)2](I2‚I3)2 (both occupancies of the disordered protons are shown); (b) [tppz(H)2](B(C6H5)4)2. Table 4. Geometric Parameters for the [tppz(H)2]2+ Cation Cpyz-Cpyz (Å) Cpyz-Cpyr (Å) Npyz-Cpyz-Cpyz (°) Npyz-Cpyz-Cpyr (°) Cpyz-Cpyz-Cpyr (°) Npyr-Cpyr-Cpyz (°) Npyr-H‚‚‚Npyr′ (Å)

[tppz(H)2](I2‚I3)2

[tppz(H)2](B(C6H5)4)211,a

1.441(11) 1.484(7) 117.0(4) 113.2(5) 129.6(4) 120.8(6) 2.563(5)

1.42 1.50 119 111 130 119 2.53

a All derived parameters reported for this structure were assigned only general uncertainties of ((0.01 Å) and ((1°).

Figure 9. Crystal packing of [tppz(H)2](I2‚I3)2 viewed down the a-axis, parallel to the polyiodide chains (origin is the upper, left, rear corner; +x is out; +y is down; +z is right).

As in the tetraphenylborate salt, tppz acts as a proton sponge in becoming diprotonated; however, the conformation of the dication is quite different with polyiodide as counterion. Severe distortion in the tppz molecule must occur to accommodate the intramolecular N-H‚‚ ‚N hydrogen bond, as a planar conformation would require an impossibly short N‚‚‚N contact of ∼1.4 Å. In the tetraphenylborate salt, the dication takes on a “bowed” shape in which the two bridges are displaced to opposite sides of the pyrazine ring plane. For each such bridge, the pyridine rings are displaced to the same side of the pyrazine plane. In the polyiodide salt, the dication is “twisted” about the N- - -N vector of the pyrazine ring. The pyridine rings involved in each bridge are displaced to opposite sides of the pyrazine ring plane. A side view of the two forms of the dication is given in Figure 10, and a comparison of selected geometric parameters is given in Table 4. Although significantly distorted, particularly at the angles involving the bridgehead carbons of the pyrazine and pyridine groups, the bonding distances and angles are essentially

identical for the two forms. In addition, computational analysis based on DFT calculations indicates that the energy difference between the two conformations is ∼0.1 kcal/mol, which is less than the error in these calculations. The major differences between the two conformations of the dication involve the planarity of the pyrazine ring, and the dihedral angle between the hydrogen bond bridged pyridine ring planes (refer to Table 2). In the bowed form, the pyridine ring has a maximum displacement of only 0.0004 Å, whereas in the twisted form, the pyrazine carbon is puckered to opposite sides of the mean plane by 0.101(6) Å. The dihedral angle between pyridine ring planes is much greater in the bowed form than in the twisted (35.4° versus 11.1(2)°). Although these distorted forms of tppz cannot be directly related to the unconstrained conformations of tppz discussed before, the origin of the two diprotonated forms can be traced back to the conformation of the free ligand prior to protonation. The bowed form is associated with the 3XNXN conformation found in the tetragonal polymorph of tppz in which the pyridyl nitrogen atoms are on the same side of the pyrazine ring plane. The twisted form is associated with the 4NNNN conformation found in the tppz‚TIE complex and predicted to

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Figure 11. Thermal ellipsoid plot (50% probability) of [tppz(H)4](Br)4‚2H2O (letters indicate symmetry operator: A, 1 - x, 1 y, 1 - z).

Figure 13. Crystal packing of [tppz(H)4](I)2(I3)2 viewed down the a-axis, parallel to the segregated cation-anion stacks (origin is the upper, left, rear corner; +x is out; +y is down; +z is right).

Figure 12. Thermal ellipsoid plot (50% probability) of [tppz(H)4](I)2(I3)2 (letters indicate symmetry operator: A, 1 - x, 1 - y, 1 - z).

be favored in the gas phase. Although this presents an interesting mental exercise, our computational analysis indicates that the barrier to interconversion between these two forms is very small, and the conformer found is probably more related to subtle packing effects of the counterion present. Tetraprotonated Salts of tppz. The tetraprotonated tppz can only form intermolecular hydrogen bonds. In a chloride salt, [tppz(H)4]4+ interacts directly with two chloride anions, each of which bridges two pyridyl rings through two N-H‚‚‚Cl interactions to give a [tppz(H2Cl)2]2+ cation with a 5XXXX conformation. We have found that the bromide salt, [tppz(H)4](Br)4‚2H2O, shown in Figure 11 is isomorphous with the chloride derivative, crystallizing in the monoclinic space group, P21/c, with the [tppz(H2Br)2]2+ cation situated upon an inversion center at (1/2, 1/2, 1/2) and two bromide ions and one water molecule occupying general positions. The N‚‚‚Br distances within the cation are 3.187(6) and 3.201(6) Å. The water molecules link the additional bromide anions into chains through O-H‚‚‚Br hydrogen bonding with O‚‚‚Br distances of 3.276(8) and 3.369(8) Å, and there are no significant interactions between these chains and the cations. The iodide salt, [tppz(H)4](I)2(I3)2, has an identical [tppz(H2I)2]2+ cation (see Figure 12), and also crystallizes in space group, P21/c, with the cation situated upon an inversion center at (1/2, 1/2, 1/2). The N‚‚‚I distances within the cation are 3.459(5) and 3.488(5) Å.

But while the lighter halide salts incorporate water through hydrogen bonding with the two additional anions, in the iodide salt diiodine molecules are incorporated to form triiodide anions, which are linear and slightly asymmetric (I2-I3 and I3-I4 distances of 2.9543(8) and 2.8826(8) Å, respectively). There are no significant interactions between triiodide anions or between the triiodide anions and the cation in the crystal packing (Figure 13). Summary The complexes described above illustrate the enormous versatility of tppz as a multisite donor for crystal design through halogen- and hydrogen-bonding interactions. Complexation of tppz with iodine in two different stoichiometric ratios stabilizes conformations of the donor which are related to one or the other of the polymorphic forms of the free molecule. In each of these complexes, the iodine acts as a removable structural auxiliary which steers the solid-state decomposition as it follows Ostwald’s Rule of Stages. This phenomenon provides a facile and solvent-free process for the isolation and interconversion of the polymorphic forms of tppz. The organoiodide, TIE, stabilizes a new conformation of tppz, very similar to that predicted to be favored in the gas phase, but decomposition of this complex leads to the stable monoclinic polymorph. The salts of di- and tetraprotonated tppz provide evidence of the preference of iodine for halogen bonding over hydrogen bonding. The proton sponge behavior exhibited in the diprotonated cation with polyiodide counterions is identical to that observed with noncoordinating tetraphenylborate anions, although a new conformation of the cation is revealed. Tetraprotonation provides such a strong driving force for hydrogen bond formation that a bare iodide anion is captured, but the remaining anions preferentially associate with iodine molecules as acceptors for triiodide formation, rather than associate with water molecules through hydrogen bonding as is

Complexes of 2,3,5,6-Tetra(2′-pyridyl)pyrazine

favored with the lighter halogens. As we continue our investigation of tppz as an extraordinary tool for structure design, we will pursue additional examples of complex decomposition to isolate metastable polymorphs. We will also focus on the use of polyiodides as noncoordinating anions, for example, to encourage formation of intramolecular hydrogen bonds within a cation and will seek additional examples of the diprotonated tppz as a proton sponge. Acknowledgment. This material is based upon work supported by the National Science Foundation under Grant No. CHE-0203402. The authors are grateful to Professor Michael D. Ward of the University of Minnesota for helpful discussion. Supporting Information Available: X-ray crystallographic information files (CIF) and rotatable three-dimensional images in Quicktime format are available for all five compounds. This material is available free of charge via the Internet at http://pubs.acs.org.

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